Mobile mining machines such as mobile transfer conveyors, loaders, and continuous miners have an endless conveyor element such as a belt or chain trained for orbital movement in conveying and return runs between inbye and outbye reversing members such as pulleys or sprockets.
The outbye pulley or sprocket is almost always mounted on a vertically movable boom, to adjust its discharge height. The inbye pulley or sprocket may also be mounted on a vertically movable boom, this being the case for mobile transfer conveyors in which both receiving and discharge booms extend from the main frame.
These machines have a short wheel or crawler base and a relatively long conveyor, often with substantial fore and aft overhang. A small dipping movement of the base when being trammed or towed along an uneven mine bottom causes a much larger vertical movement of the ends of the machine, including one or both ends of the conveyor booms. This can cause the end of the conveyor boom to strike the top or bottom with considerable force and damage the conveyor or wedge or hang up the machine so the conveyor has to be raised or lowered to free it.
Further, these conveyor booms are raised and lowered by powerful hydraulic jacks operating under several hundred or even several thousand pounds per square inch. For maximum production and conveying capacity, these machines generally use up most of the available head room and a common problem is hang up of one end or the other with the top or bottom when the operator makes a tilting or leveling adjustment. Again the machine has to be raised or lowered at one end to free it.